During the past year, we have continued to study a group of monoclonal antibodies prepared by immunizing rats with mouse brain. Thirteen different antibodies have been produced. So far, seven of these have been partially analyzed. Four of them react with multipotential stem cells (CFUs). Only one of the monoclonal antibodies which kill CFUs also reacts with prothymocytes. Another of these reagents reacts with all B cells. Another antibrain monoclonal antibody also reacts with B cells and is mitogenic for these cells. Two other antibodies have been extensively characterized. These react with, and are cytotoxic for, prothymocytes (AE3.38 and BG2.17). During the past year, some progress has been made with growing T lymphocytes from the lymph nodes of immunized mice. Much of the work this past year has been on the chemical synthesis of the photoactive reagent diazidobenzoyl cystine, an aryl azide containing both a radioactive isotope label and a disulfide capable of interacting with proteins. This reagent has been attached to a variety of proteins and groups of mice have been immunized with each conjugate. Hybridoma cells were prepared by fusion of spleen cells from one of these mice with 3PU-1 myeloma cells, resulting in a number of colonies of antibody-secreting cells. A limit-dilution was carried out on each of these colonies and produced four vigorously growing clones of hybrid cells secreting antibody. These have been recloned and used to generate ascites in mice. The resulting antibody has been with the photoaffinity-labeled antigen. T cells from the mice immunized with this antigen also have been placed in culture, and lines which seem to have reactivity with diABC are being maintained. During the next year we plan to: (1)\identify the proteins which characterize the CFUs subsets and prothymocytes; (2)\produce highly enriched populations of CFUs and prothymocytes; and (3)\produce T cell hybridomas with reactivity for diABC and use these to isolate an antigen-binding material for these cells.